It has been known that in an engine mounted in a motorcycle or the like, if the gas pressure in a combustion chamber increases when an engine rotation speed decreases to a speed lower than an idle rotation speed, there is a possibility that the engine will be reversely rotated.
Japanese Unexamined Patent Application Publication No. 2006-207565 discloses an engine ignition control device for preventing the reverse rotation of the engine by forcibly making the engine misfire when the revolution speed of the engine becomes lower than a threshold value determined according to a present engine load. However, it would be preferable to prevent the gas pressure itself in the combustion chamber from increasing than to forcibly make the engine misfire, because useless fuel consumption is decreased.